Retail Design Pattern Analysis and Refactoring

# Snippet 1: Cart Component
# Refactored from Singleton pattern to Repository pattern
# Carts are now stored separately by cart_id so each user can have their own cart instead of sharing one cart across the entire application.

class Cart:
    # cart_id keeps each customer's cart separate.
    def __init__(self, cart_id):
        self.cart_id = cart_id
        self.items = []

    # Same add item behavior as the original code.
    def add_item(self, item, quantity):
        self.items.append((item, quantity))
        print(f"Added {quantity} {item}(s) to cart.")


class CartRepository:
    # Repository stores Cart objects by cart_id.
    def __init__(self):
        self._carts = {}

    # Return an existing cart or create one if it does not exist.
    def get_cart(self, cart_id):
        if cart_id not in self._carts:
            self._carts[cart_id] = Cart(cart_id)
        return self._carts[cart_id]




# Snippet 2: Payment Component
# Refactored from Factory pattern to Strategy pattern.
# The original code used PaymentProcessorFactory and if/elif checks to pick a processor.
# This version lets the calling code pass in the payment strategy instead.
# New payment methods can be added as new strategy classes instead of more if/elif logic.

# Base strategy abstraction for payment methods.
class PaymentStrategy:
    def process_payment(self, amount):
        raise NotImplementedError("Each payment strategy must define process_payment().")


# Handles credit card payments.
class CreditCardPaymentStrategy(PaymentStrategy):
    def process_payment(self, amount):
        print(f"Processing {amount} via Credit Card.")


# Handles PayPal payments.
class PayPalPaymentStrategy(PaymentStrategy):
    def process_payment(self, amount):
        print(f"Processing {amount} via PayPal.")


class PaymentProcessor:
    def __init__(self, strategy):
        self.strategy = strategy